This invention relates generally to ultrasonic transceivers, and more particularly to a transponder capable of transmitting and receiving ultrasonic signals for gauging liquid level.
In ultrasonic liquid level measurement, a transmitter at a measuring station emits ultrasonic pulses which are directed toward the surface of the liquid whose level is to be gauged, the transmitted pulses passing through the atmospheric region above the liquid surface. The echo pulses reflected by the liquid surface are picked up by a receiver at the same station, the elapsed time between the transmission and reception of pulses being electronically measured and indicated. Since the elapsed time is proportional to the straight line distance between the measuring station and the liquid surface, it serves as an accurate index to liquid level.
In practice, a liquid level gauge may make use of separate transducers, one for transmission and the other for reception. Or one may employ an ultrasonic transceiver which is capable of both transmitting and receiving ultrasonic signals. The primary concern of the present invention is with transceivers of the latter type.
When an ultrasonic transducer is activated by a short burst of energy to generate an ultrasonic pulse, it continues to ring after the energy is removed. With exisiting types of transceivers, the same transducer cannot always be used both for transmission and reception, in that when measuring short distances between the measuring station and the liquid surface, the vibratory activity or ringing of the transducer at the termination of transmission is not damped with sufficient rapidity to render the same transducer operative as a receiver at the instant an echo pulse reflected from the liquid surface arrives.
In order, therefore, for a transceiver to function properly, the damping must be such as to quickly terminate ringing of the transducer, which damping must be effective through a broad temperature range such as that encountered in an outdoor environment. The shorter the distance between the station and the liquid level, the greater the need for fast damping.
The distance between the measuring station and the surface of the liquid being gauged is referred to as the "dead" distance this dead distance having a minimum value when the liquid reaches its maximum level. It is desirable in liquid level gauging that the dead distance be kept as short as possible.
It has therefore been necessary in liquid level gauging, when the dead distance is short, to provide separate transducers for transmission and reception, in that the damping rate of a typical transceiver was too slow. The inherent cost disadvantages of an arrangement requiring the installation of two transducers are obvious.
In order to effect rapid damping so that the same transducer can function effectively both as a transmitter and receiver, it is known to embed the transducer in an elastomeric medium. Thus U.S. Pat. No. 4,031,503 to Minami discloses a disc-type electrostriction element affixed to a rigid body of greater thickness and suspended within a casing by a diaphragm secured to the body at a vibratory node, the casing being filled with electrically non-conductive elastomeric material.
In addition to effecting rapid damping, the elastomeric medium in the Minami arrangement serves to protectively pot the transducer so that it is not susceptible to erosion in a corrosive environment such as that encountered when taking liquid level measurments in oil wells and mine shafts.
The present invention constitutes an improvement over the Minami arrangement and makes it possible to produce a higher level of ultrasonic energy with decreased ringing, as well as more stable portion over a wide temperature range. The advantages of the present arrangement over that disclosed in the Minami patent will be spelled out in greater detail in a subsequent section of this specification which described a transceiver in accordance with the invention.